Info

where:

INT is the integer (truncation) function

Dmax is the optical density corresponding to the maximum output of the digitizer

D0 is the optical density corresponding to a digitizer value of 0

n is the number of bits of digitization

In the thresholding procedure, an observer, while viewing the mammogram, manipulates a computer trackball to select threshold gray levels that identify the edges of regions of density in the breast. The result is observed on a graphics overlay of a computer display of the image. For each image, only two relatively simple threshold decisions must be made. The first identifies the edge of the breast from the background (area outside the breast). This threshold is referred to as iEDGE, and it is illustrated for the mammogram of Fig. 2 by the dashed line. Similarly, a second threshold is selected that best outlines region(s) of mammographic density in the image, and above which all pixels are interpreted as mammographic density. This threshold is referred to as iDY. Pixels having this value are represented by the solid bright line in Fig. 2. The size of each region can be determined by counting the enclosed pixels, a process that is simplified by considering the histogram of gray-level frequencies from pixels within the breast. The histogram is constructed such that hi represents the number of pixels with gray level i (Fig. 3). The area under the histogram (summing all pixels in the histogram above iEDGE to the maximum gray-level imax), is then a measure of the projected area of the breast, A:

FIGURE 2 Interactive thresholding procedure, PD, for the quantitative determination of the proportion of the area of the projected image of the breast that represents mammographic density. The red line indicates the selected threshold for the edge of the breast; the area shaded in green indicates the selected threshold for the edge of mammographically dense tissue. See also Plate 48.

FIGURE 2 Interactive thresholding procedure, PD, for the quantitative determination of the proportion of the area of the projected image of the breast that represents mammographic density. The red line indicates the selected threshold for the edge of the breast; the area shaded in green indicates the selected threshold for the edge of mammographically dense tissue. See also Plate 48.

in the breast. The ratio of these totals defines the proportion, by area, of mammographic density in the breast, PD:

0 0

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